Radioactivity
Part 1
Radioactivity
Radioactivity refers to the particles which are emitted from nuclei
as a result of nuclear instability. Because the nucleus experiences
the intense conflict between the two strongest forces in nature, it
should not be surprising that there are many nuclear isotopes which
are unstable and emit some kind of radiation. Different isotopes of
a given element have the same atomic number but different mass numbers
since they have different numbers of neutrons. The chemical properties
of the different isotopes of an element are identical, but they will
often have great differences in nuclear stability.
The most common types of radiation are called alpha, beta, and gamma
radiation, but there are several other varieties of radioactive decay.
Radioactive decay rates are normally stated in terms of their
half-lives,
and the half-life of a given nuclear species is related to its
radiation
risk.
1 Radioactive
Half-Life
The radioactive half-life for a given radioisotope is the time for
half the radioactive nuclei in any sample to undergo radioactive decay.
After two half-lives, there will be one fourth the original sample,
after three half-lives one eight the original sample, and so forth.
The different types of radioactivity lead to different decay paths
which transmute the nuclei into other chemical elements. Examining
the amounts of the decay products makes possible radioactive dating.
Part 2
Alpha α Radioactivity
Composed of two protons and two neutrons, the alpha particle is a
nucleus of the element helium. Because of its very large mass (more
than 7000 times the mass of the beta particle) and its charge, it
has a very short range. It is not suitable for radiation therapy since
its range is less than a tenth of a millimeter inside the body. Its
main radiation hazard comes when it is ingested into the body; it
has great destructive power within its short range. In contact with
fast-growing membranes and living cells, it is positioned for maximum
damage.
Part 3
Beta β Radioactivity
Beta particles are just electrons from the nucleus, the term "beta
particle" being an historical term used in the early description
of radioactivity. The high energy electrons have greater range of
penetration than alpha particles, but still much less than gamma rays.
The radiation hazard from betas is greatest if they are ingested.
Beta emission is accompanied by the emission of an electron
anti neutrino
which shares the momentum and energy of the decay.
Part 4
Gamma γ Radioactivity
Gamma radioactivity is composed of electromagnetic rays. It is
distinguished
from x-rays only by the fact that it comes from the nucleus. Most
gamma rays are somewhat higher in energy than x-rays and therefore
are very penetrating. It is the most useful type of radiation for
medical purposes, but at the same time it is the most dangerous because
of its ability to penetrate large thicknesses of material.
Part 5
Dosage and Dosage Terminology
1
Radiation Risk
Because the energies of the particles emitted during radioactive
processes
are extremely high, nearly all such particles fall in the class of
ionizing radiation.
2
Activity of Radioactive Source
The curie (Ci) is the old standard unit for measuring the activity
of a given radioactive sample. It is equivalent to the activity of
1 gram of radium. It is formally defined by:
- 1 curie = amount of material that will produce 3.7 x 1010
nuclear decays per second.
- 1 Becquerel = amount of material which will produce 1
nuclear
decay
per second.
- 1 curie = 3.7 x 1010 becquerels.
The bequerel is the more recent SI unit for radioactive source
activity.
3
Absorbed Dose of Radiation
The rad is a unit of absorbed radiation dose in terms of the energy
actually deposited in the tissue. The rad is defined as an absorbed
dose of 0.01 joules of energy per kilogram of tissue. The more recent
SI unit is the gray, which is defined as 1joule of deposited energy
per kilogram of tissue. To assess the risk of radiation, the absorbed
dose is multiplied by the relative biological effectiveness of the
radiation to get the biological dose equivalent in rems or sieverts.
4
Biologically Effective Dose
The biologically effective dose in rems is the radiation dose in rads
multiplied by a "quality factor" which is an assessment
of the effectiveness of that particular type and energy of radiation.
For alpha particles the relative biological effectiveness (rbe) may
be as high as 20, so that one rad is equivalent to 20 rems. However,
for x-rays and gamma rays, the rbe is taken as one so that the rad
and rem are equivalent for those radiation sources. The sievert is
equal to 100 rems.
SI multiples are the millisievert (1mSv = 10-3Sv)
and
microsievert (1mSv = 10-6Sv).
An older unit of the equivalent dose is the rem (Roentgen equivalent
man); 1 Sv is equal to 100 rem. In some fields, rem and mrem continue
to be used along with Sv and mSv, unavoidably causing confusion (1
Sv = 100 rem, 10 mSv = 1 rem: it is hard to memorize, when to use
which conversion factor).
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On 31 Jan 2006, 19:51.
